Prior to the cryogenic separation of oxygen and nitrogen from air, various trace air impurities must be removed to avoid formation of solids in heat exchange equipment and resultant high pressure drops in the cryogenic process. The most obvious trace air impurities that must be removed include CO.sub.2 and water.
However, many air separation plants are in close proximity to stationary combustion sources or roadways. In these instances, there may be significant quantities (ppm levels) of nitrogen oxides present in the ambient air. These impurities could include; NO, NO.sub.2 and N.sub.2 O. These impurities could further react at low temperature to form other nitrogen oxides, including; N.sub.2 O.sub.3, N.sub.2 O.sub.4 and N.sub.2 O.sub.5. Since these materials form solids at liquid nitrogen temperatures, it is possible that their presence in ambient air could lead to freeze out problems in the cold end of the cryogenic separation plant. Therefore, it is desirable to devise an air pretreatment system which not only removes CO.sub.2 and water, but removes nitrogen oxides as well.
The first reference to use of a pressure swing adsorption (PSA) drier is U.S. Pat. No. 2,944,627. Using purge to feed ratios greater than 1.0 on an actual volume of gas basis, it was found that using an alumina adsorbent, the product air was devoid of water, CO.sub.2 and oil vapor. No mention of acetylene or nitrogen oxides is made.
German Patent Publication DE 3,045,451 (1981) describes a PSA process which operates at 5-10.degree. C., 880 KPa adsorption pressure and 98 KPa regeneration pressure. Feed air is passed through a layer of 13X particles to remove the bulk of water vapor and CO.sub.2 and then through a final layer of alumina for final clean-up. The alumina section can constitute 20-80% of the bed volume. The bed layering is claimed to reduce formation of "cold spots" in the adsorbent beds. Nitrogen oxide removal is not mentioned.
U.S. Pat. No. 4,711,645 describes a PSA process for removal of water and CO.sub.2 utilizing alumina for water removal followed by a zeolite for CO.sub.2 removal. It is claimed that the use of alumina for water removal allows adsorption at a lower temperature (due to its lower heat of adsorption) which increases the capacity of the zeolite for CO.sub.2.
U.S. Pat. No. 4,249,915 describes a PSA process where water and CO.sub.2 are removed from atmospheric air by adsorption in two separate beds. The moisture-laden bed is regenerated by PSA in a relatively short operating cycle, while the CO.sub.2 -laden bed is thermally regenerated at longer time intervals. Removal of nitrogen oxides is not mentioned.
U.S. Pat. No. 5,232,474 teaches a PSA process for pre-purification of air using an initial layer of alumina which comprises 70 to 100% of the bed volume, with the remaining layer, if present, a suitable zeolite. Thus, alumina may be the sole adsorbent present. The benefit of using a solely alumina bed is that it substantially reduces the cold zone that develops in a bed of zeolite during desorption. Since zeolites adsorb significantly more air than alumina, rapid desorption of air from the zeolite results in an acute temperature drop in the bed. The low temperature at which desorption occurs increases the amount of purge gas needed for regeneration. A further benefit of the all alumina bed is less void gas losses, since zeolites adsorb, and hence desorb, more air during blowdown.
E.P. Patent 1 586 961 describes a PSA process for the removal of CO.sub.2, water and acetylene from air. Acetylene removal is accomplished with the use of other adsorbents in the bed or further adsorption beds.
EP 0 449 576 A1 teaches using four discrete adsorbent layers, 2 of alumina followed by two more of zeolite, for front-end pre-purification.
There is some limited art which teaches hydrocarbon removal in pre-purification units. For example, DE 37 02 190 A1 discloses the removal of C.sub.2 H.sub.2, C.sub.2 H.sub.4 and C.sub.3 H.sub.6 in addition to CO.sub.2 and water.
The prior art has used alumina and other zeolites in layered pretreatment PSA beds upstream of cryogenic air separation. However, the prior art was not aware of instances where nitrogen oxides might exist in elevated levels sufficient to cause process problems in cryogenic air separation where the nitrogen oxides could freeze out and form solids in the process lines which would impair or stop the operation of the process. The present invention has discovered this problem which exists in those areas where nearby combustion sources elevate the nitrogen oxide levels of ambient air and provides a unique solution to the removal of the elevated levels of nitrogen oxides using low energy intensive processing and low capital cost systems, as will be described in greater detail below.